Fibrinogen, the main structural protein in the blood responsible for the formation of clots, exists as a dimer of three polypeptide chains; the Aα (66.5 kD), Bβ (52 kD) and γ (46.5 kD) are linked through 29 disulphide bonds. The addition of asparagine-linked carbohydrates to the Bβ and γ chains results in a molecule with a molecular weight of 340 kD. Fibrinogen has a trinodal structure, a central nodule, termed the E domain, contains the amino-termini of all 6 chains including the fibrinopeptides (Fp) whereas the two distal nodules termed D domains contain the carboxy-termini of the Aα, Bβ and γ chains. Fibrinogen is proteolytically cleaved at the amino terminus of the Aα and Bβ chains releasing fibrinopeptides A and B (FpA & FpB) and converted to fibrin monomer by thrombin, a serine protease that is converted from its inactive form by Factor Xa. The resultant fibrin monomers non-covalently assemble into protofibrils by DE contacts on neighboring fibrin molecules.
Fibrinogen is naturally subject to phosphorylation, sulfation, and glycosylation. Glycosylation is a complex process of post-translational modification and has important functions in secretion, protein folding, immunogenicity and clearance of glycoproteins from the bloodstream. Glycoprotein glycans are mainly attached to proteins via an N- or an O-glycosidic bond. N-linked glycosylation occurs to the side-chain group of an Asparagine (Asn) residue, whereas O-linked glycosylation occurs to the side-chain group of Ser or Thr. It is well established that human fibrinogen contains glycans linked to Asn residues in the Bβ and gamma chains in the Asn-Arg-Thr (Asn at position 364) and Asn-Lys-Thr (Asn at position 52) sequence, respectively (Topfer-Peterson, 1976, Hoppe Seylers Z Physiol Chem 357:1509; Blomback, 1973, J. Biol. Chem 248:5806).
The current standard of care for patients with fibrinogen deficiencies involves replacement therapy with human fibrinogen containing preparations such as plasma-derived fibrinogen (pdFIB), or fresh-frozen plasma and cryoprecipitates, both of which contain pdFIB.
Although therapy with human fibrinogen containing preparations can be effective at controlling bleeding, pathologic thromboses with serious sequelae are well-known complications of such infusions (Lak, Br J Haematol, 1999 October; 107(1):204-6). Often the abnormal clots can occur after the initial bleeding episode has been treated (Pati, Surg. Neurol. 2008 Feb. 22; Matsumoto, Haemophilia 2008 Jan.; 14(1):153-6). In the broader setting of hereditary bleeding disorders, in which this phenomenon has also been observed, excessive levels of replaced clotting factors for prolonged durations have been thought to contribute to these events (Franchini, Thromb. Haemost. 2004 May; 91(5):1053-5). In the case of bleeding due to a deficiency of fibrinogen, replacement therapy with pdFIB has been directly implicated as a contributing factor in the etiology of pathologic thromboses (Kreuz, Transfus. Apher. Sci. 2005 June; 32(3):247-53).